Magneto-optical material, faraday rotator, and optical isolator

ABSTRACT

It is an object of the present invention to provide a magneto-optical material containing as a main component an oxide that includes a terbium oxide and having a large Verdet constant at a wavelength in the 1.06 μm region (0.9 to 1.1 μm) and high transparency, and to provide a small-sized optical isolator suitably used in a fiber laser for a processing machine. 
     The magneto-optical material of the present invention contains an oxide represented by Formula (I) below at a content of at least 99 wt %. 
       (Tb x R 1-x ) 2 O 3   (I)
 
     (In Formula (I), x satisfies 0.4≦x≦1.0 and R includes at least one element selected from the group consisting of scandium, yttrium, and lanthanoid elements other than terbium.)

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a magneto-optical material, a Faradayrotator, and an optical isolator. More particularly, it relates to amagneto-optical material that is suitable for forming a magneto-opticaldevice such as an optical isolator, and to a magneto-optical devicecomprising the magneto-optical material.

2. Background Art

Recently, with the development of laser processing machines,magneto-optical devices utilizing the interaction of light and magnetismhave become much noticed. One of them is an isolator, which is asfollows: when the light oscillated from a laser source is reflected bythe optical system on the way and is returned to the light source, thenit disturbs the light oscillated from the laser source thereby providingan unstable oscillation state; and the isolator prevents the phenomenon.Accordingly, based on the action, the optical isolator is arrangedbetween a laser source and an optical member and is utilized sotherebetween.

The optical isolator comprises three parts, a Faraday rotator, apolarizer arranged on the light-incoming side of the Faraday rotator,and an analyzer arranged on the light-outgoing side of the Faradayrotator. The optical isolator functions based on its property that whenlight comes in the Faraday rotator thereof under the condition where amagnetic field is applied to the Faraday rotator in the directionparallel to the light running direction, then the plane of polarizationrotates in the Faraday rotator, or that is, the Faraday effect.Specifically, of the incident light, the light having the same plane ofpolarization as that of the polarizer is, after having passed throughthe polarizer, introduced into the Faraday rotator. The light is rotatedby +45 degrees relative to the light running direction in the Faradayrotator, and then goes out of the isolator.

As opposed to this, when the light returning into the Faraday rotator inthe direction opposite to the incident direction first passes throughthe analyzer, only the component of the light having the same plane ofpolarization as that of the analyzer passes through the analyzer and isintroduced into the Faraday rotator. Then, in the Faraday rotator, theplane of polarization of the returning light is further rotated by +45degrees additionally to the initial +45 degrees, and therefore, theplane of polarization thereof is right-angled by +90 degrees to thepolarizer, and the returning light could not pass through the polarizer.

It is necessary that the material to be used for the Faraday rotator ofthe optical isolator mentioned above has a large Faraday effect and hashigh transmittance at the wavelength for its use.

Recently, as laser processing machines, many devices with fiber laserhave become much utilized. The oscillation wavelength of the laser is0.9 to 1.1 μm, and as the material having a large Faraday effect andhigh transmittance at the wavelength, used are terbium gallium garnetsingle crystal (abbreviation: TGG), terbium aluminium garnet singlecrystal (abbreviation: TAG), etc. (See Patent Document 1).

The Faraday rotation angle θ is represented by Formula (A) below:

θ=V×H×L  (A)

In Formula (A), V is a Verdet constant and is a constant determined bythe material of the Faraday rotator; H is the density of magnetic flux;and L is the length of the Faraday rotator. For use as an opticalisolator, L is so determined that θ=45 degrees.

Accordingly, the factor to determine the size of the optical isolatorincludes the Verdet constant and the density of magnetic flux. TheVerdet constant of terbium gallium garnet single crystal is 0.13min/(Oe·cm), the Verdet constant of terbium aluminium garnet singlecrystal is 0.14 min/(Oe·cm). In case where the single crystal of thetype is used and when the density of magnetic flux is 10,000 Oe, then itis necessary that the length of the Faraday rotator is 20 to 25 mm inorder to rotate the plane of polarization of the incident light by +45degrees. Accordingly, the Faraday rotator having that size must be usedand a polarizer and an analyzer formed of, for example, a rutile crystalmust be fitted to both sides of the Faraday rotator, or that is, thesize of the optical isolator will have to be at least about 70 mm. Fordownsizing the module of fiber laser, the optical isolator must bedownsized, and therefore, a material capable of shortening itsconstitutive member, Faraday rotator must be developed.

On the other hand, as a material having a large Faraday rotation angleper the unit length, there is known iron (Fe)-containing yttrium irongarnet (commonly known as YIG) single crystal (see Patent Document 2);however, the material has a large light absorption at a wavelength of0.9 μm and the absorption has some influence on a wavelength range of0.9 to 1.1 μm; and therefore, the material is unsuitable for use in thatrange.

Furthermore, Patent Document 3 discloses a terbium-containing glass anda magneto-optical device employing same. This glass also has a limit forthe terbium content.

PRIOR ART DOCUMENTS Patent Documents

-   (Patent Document 1) JP-A-7-89797 (JP-A denotes a Japanese unexamined    patent publication application)-   (Patent Document 2) JP-A-2000-266947-   (Patent Document 3) JP-A-2008-230907

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a magneto-opticalmaterial including terbium oxide, which has a large Verdet constant in awavelength range around 1.06 μm (0.9 to 1.1 μm) and has hightransmittance.

Another object of the present invention is to provide a downsizedoptical isolator favorable for use in fiber lasers for processingmachines.

Means for Solving the Problems

The object of the present invention has been attained by means describedin <1>, <9>, and <10> below. They are described below together with <2>to <8>, which are preferred embodiments.

<1> A magneto-optical material comprising an oxide represented byFormula (I) below at a content of at least 99 wt %,

(Tb_(x)R_(1-x))₂O₃  (I)

(in Formula (I), x satisfies 0.4≦x≦1.0 and R comprises at least oneelement selected from the group consisting of scandium, yttrium, andlanthanoid elements other than terbium),<2> the magneto-optical material according to <1>, wherein it has aVerdet constant at a wavelength of 1.06 μm of at least 0.18 min/(Oe·cm),a transmittance for an optical path length of 3 mm at a wavelength of1.06 μm of at least 70%, and an extinction ratio at an optical pathlength of 3 mm of at least 25 dB,<3> the magneto-optical material according to <1> or <2>, wherein inFormula (I) above, R is selected from the group consisting of scandium,yttrium, lanthanum, europium, gadolinium, ytterbium, holmium, andlutetium,<4> the magneto-optical material according to any one of <1> to <3>,wherein it comprises an oxide of an alkaline earth metal, Group 11element, Group 12 element, Group 13 element, Group 14 element, Group 15element, Group 16 element, Group 4 element, Group 5 element, or Group 6element or a compound of a Group 17 element at a content of at least0.00001 wt % but no greater than 1.0 wt %,<5> the magneto-optical material according to any one of <1> to <4>,wherein it comprises an oxide of an alkaline earth metal at a content ofat least 0.00001 wt % but no greater than 1.0 wt %,<6> the magneto-optical material according to any one of <1> to <5>,wherein it is a single crystal,<7> the magneto-optical material according to <6>, wherein it isproduced by a production method selected from the group consisting of afloating zone melting method, a micro-pulling-down method, a pulling upmethod, a skull melting method, the Bridgman method, the Bernoullimethod, and the EFG method,<8> the magneto-optical material according to any one of <1> to <5>,wherein it is a ceramic,<9> a Faraday rotator for a wavelength of at least 0.40 μm but nogreater than 1.2 μm employing the magneto-optical material according toany one of <1> to <8>, and<10> an optical isolator comprising the Faraday rotator according to <9>and a polarizing material placed in front of and to the rear of theFaraday rotator.

Effects of the Invention

According to the present invention, there has been provided amagneto-optical material including terbium oxide, which has a largeVerdet constant in a wavelength range around 1.06 mm and which has hightransparency. Also according to the present invention, there has beenprovided a downsized optical isolator favorable for use as fiber lasersfor processing machines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing one example of an apparatussuitable for use for a floating zone method.

FIG. 2 is an explanatory view showing one example of a micro-pull downmethod.

FIG. 3 is a schematic cross-sectional view showing one example of anoptical isolator, a magneto-optical device having a Faraday rotator asthe magneto-optical element thereof.

MODES FOR CARRYING OUT THE INVENTION

The magneto-optical material of the present invention comprises an oxiderepresented by Formula (I) below at a content of at least 99 wt %.

(Tb_(x)R_(1-x))₂O₃  (I)

wherein In Formula (I), x satisfies 0.4≦x≦1.0 and R comprises at leastone element selected from the group consisting of scandium, yttrium, andlanthanoid elements other than terbium.

This magneto-optical material preferably has a Verdet constant at awavelength of 1.06 μm of at least 0.18 min/(Oe·cm) and a transmittancefor an optical path length of 3 mm at a wavelength of 1.06 μm of atleast 70%, and an extinction ratio at an optical path length of 3 mm ofat least 25 dB.

The present inventors have noted that terbium, which is a paramagneticelement, and an oxide thereof have high transparency at a wavelength of1.06 μm, and the feasibility of obtaining a large Verdet constant atthat wavelength has been intensively investigated. As a result, it hasbeen found that by producing a magneto-optical material from an oxidecomprising terbium oxide at a molar ratio-based content of at least 40%and a rare-earth that is transparent at a wavelength of 1.06 μm, thatis, R in Formula (I) above is for example selected from the groupconsisting of scandium, yttrium, lanthanum, europium, gadolinium,ytterbium, holmium, and lutetium, a Verdet constant at a wavelength of1.06 μm shows a value of at least 0.18 min/(Oe·cm), and the presentinvention has thus been accomplished.

Terbium (Tb) is a paramagnetic element, and since terbium oxide is acompound having a light transmittance of at least 70% for an opticalpath length of 3 mm at a wavelength of 1.06 μm, it is the most suitableelement for use as an isolator in this wavelength region. Therefore,producing a compound of Formula (I) that has as much terbium as possiblecan result in obtaining a compound having a large Verdet constant at1.06 μm and an increased Faraday rotation angle. Furthermore, in orderto produce a compound having high transparency at a wavelength of 1.06μm, it is preferable for another constituent element to have hightransparency in that wavelength region, and the most suitable compoundis an oxide of an element having a light transmittance of at least 70%for an optical path length of 3 mm at a wavelength of 1.06 μm.

The magneto-optical material of the present invention preferably has anextinction ratio for an optical path length of 3 mm of at least 25 dB.It is preferable for it to have this extinction ratio from the viewpointof enabling an optical isolator having optical characteristics with highisolation to be produced.

The extinction ratio may be measured at a wavelength of 1.06 μm inaccordance with a standard method. Measurement conditions are 25±10° C.,and measurement is carried out in the atmosphere.

In the present invention, the Verdet constant may be measured inaccordance with a standard method, and measurement conditions are notparticularly limited. Specifically, an oxide having a predeterminedthickness is cut out and finished by mirror polishing, a Faraday rotatoris set in a permanent magnet having a known magnetic flux density, andthe Verdet constant at a wavelength of 1.06 μm is measured. Furthermore,measurement conditions are 25±10° C., and measurement is carried out inthe atmosphere.

On the other hand, an oxide containing terbium in the largest amount andhaving a highest Verdet constant is terbium oxide of itself. Theinventors tried growing the single crystal according to a flowing zonemethod; however, after the growth, the crystal cracked in cooling.Though the detailed reason is not clear as yet, it is considered thatterbium oxide could include two morphologies of Tb₂O₃ where Tb istrivalent and TbO₂ where Tb is tetravalent, and during cooling, theoxide would undergo phase transition to crack.

Accordingly, the inventors have investigated a solid solution of terbiumoxide with any other oxide which has the same crystal structure as thatof terbium oxide, which comprises the same rare earth element group,which is stable when its oxidation valence is trivalent and which hashigh transparency at a wavelength of 1.06 mm. Its choices includescandium, yttrium, lanthanum, europium, gadolinium, ytterbium, holmiumand lutetium; and the inventors have known that a solid solution of anoxide of that metal and terbium oxide is suitable.

Further, in the solid solution, the concentration of terbium oxide canbe changed freely in some degree. Accordingly, with varying theconcentration of terbium oxide therein, crystals were produced accordingto a floating zone method, and the Verdet constant of the crystals wasmeasured. As a result, it has been found that, when the ratio by mol ofterbium oxide is at least 40%, then the Verdet constant of the solidsolution at a wavelength of 1.06 mm could be at least 0.18 min/(Oe·cm).

In addition, the inventors have further found that when the solidsolution is analyzed through X-ray powder diffractiometry for thecrystal structure thereof, then terbium oxide and other rare earthoxides mentioned above are the same cubic crystals, and therefore thesolid solution is also the same cubic crystal.

In this embodiment, “solid solution” means that terbium existing at thelattice point of the crystal layer of terbium oxide of the startingpowder is quite irregularly substituted with any other element (forexample, yttrium, etc.). Accordingly, this includes, single crystals,polycrystals, and polycrystalline ceramics produced through sintering,etc.

The present invention is described in more detail hereinunder.

In the present invention, the wording “A to B” indicating the numericalrange means “at least A but no greater than B” unless otherwisespecifically indicated. In other words, the wording means the numericaldata including the end points A and B.

(Oxide Represented by Formula (I)) (Oxide Represented by Formula (I))

The magneto-optical material of the present invention comprises an oxiderepresented by Formula (I) as a main component, that is, at a content ofat least 99 wt %.

(Tb_(x)R_(1-x))₂O₃  (I)

(In Formula (I), x satisfies 0.4≦x≦1.0 and R comprises at least oneelement selected from the group consisting of scandium (Sc), yttrium(Y), and lanthanoid elements other than terbium (preferably lanthanum(La), europium (Eu), gadolinium (Gd), ytterbium (Yb), holmium (Ho), andlutetium (Lu)).)

In Formula (I), R is not specifically defined so far as it contains atleast one element selected from a group consisting of scandium, yttrium,lanthanum, europium, gadolinium, ytterbium, holmium and lutetium, andmay contain any other element. Examples of the other element are erbiumand thulium.

The content of the other element is preferably no greater than 50 partsby weight relative to 100 parts by weight of the total amount of R, morepreferably no greater than 10 parts by weight. Yet more preferably, thecontent of the other element is 0, that is, R is an element aloneselected from a group consisting of scandium, yttrium, lanthanum,europium, gadolinium, ytterbium, holmium and lutetium, not containingany other element.

R may be a single element, or the formula may contain different R's inany desired ratio with any specific limitation thereon.

Among them, yttrium, gadolinium and lutetium are preferred for R fromthe viewpoint that the starting materials are easily available; andyttrium is more preferred.

In Formula (I), x is 0.4 to 1.0. Specifically, the oxide represented byFormula (I) contains at least 40 mol % of Tb₂O₃ as a ratio by mol.

In Formula (I), when x is less than 0.4, the oxide could not have a highVerdet constant.

Preferably, x is at least 0.4 but less than 1.0, more preferably 0.4 to0.8, yet more preferably 0.45 to 0.75. When x falls within the aboverange, it is preferable since the oxide has a high Verdet constant andsince the oxide is excellent in transparency. In particular, when x isno greater than 0.8, it is preferable since the crystal is, after grown,prevented from cracking during cooling, and since the crystal isprevented from being clouded.

(Magneto-Optical Material)

The magneto-optical material contains an oxide represented by Formula(I) (hereinafter, also called an ‘oxide of the present invention’) asthe main component thereof.

Specifically, the magneto-optical material of the present invention maybe good to contain an oxide represented by Formula (I) as the maincomponent thereof, and may contain any other component as the accessoryconstituent. In other words, the oxide of the present invention containsan oxide represented by Formula (I) as the main component and maycontain any other component (any other oxide, etc.).

The wording “contain an oxide as the main component” means that theoxide of the present invention contains an oxide represented by Formula(I) in an amount of at least 50 wt %. Preferably, the content of theoxide represented by Formula (I) is at least 80 wt %, more preferably atleast 90 wt %, yet more preferably at least 99 wt %, particularlypreferably at least 99.9 wt %, most preferably at least 99.99 wt %.

The other component that the oxide of the present invention may compriseis preferably a metal oxide or compound selected from the groupconsisting of an oxide of an alkaline earth metal, a Group 11 element, aGroup 12 element, an oxide of a Group 13 element, an oxide of a Group 14element, a Group 15 element, in addition oxides of a Group 4 element, aGroup 5 element (V, Nb, Ta, etc.), and a Group 6 element (Mo, W, etc.),and a compound of a Group 17 element. As the Group 17 element, F, Cl,and Br are preferable, and F is more preferable, and as the compound ofa Group 17 element YF₃ and MgF₂ can be cited as examples.

The oxide of the present invention preferably comprises one or more ofan oxide of an alkaline earth metal, Group 13 element, Group 14 element,Group 4 element, Group 5 element (V, Nb, Ta, etc.), or Group 6 element(Mo, W, etc.) or a compound of a Group 17 element at a content of atleast 0.000001 wt % but no greater than 1.0 wt %.

The content of these oxides is preferably at least 0.00001 wt % but nogreater than 1.0 wt % relative to the oxide of the present invention,and more preferably 0.0001 to 0.1 wt %.

Specific examples of the oxide of an alkaline earth metal includemagnesium oxide, strontium oxide, and barium oxide, specific examples ofan oxide of a Group 11 element include copper oxide and silver oxide,specific examples of an oxide of a Group 12 element include zinc oxideand cadmium oxide, specific examples of the oxide of a Group 13 elementinclude aluminum oxide (alumina) and gallium oxide, specific examples ofthe oxide of a Group 14 element include silicon oxide, germanium oxide,and tin oxide, specific examples of an oxide of a Group 15 elementinclude bismuth oxide, and specific examples of the oxide of a Group 4element include titanium oxide, zirconium oxide, and hafnium oxide.

The metal oxide may be added, for example, as a dopant to be added insingle crystal formation, or as a residue of the sintering promoteradded in ceramic production.

As the dopant to be added in single crystal formation, preferred is analkaline earth metal oxide, more preferred are magnesium oxide,strontium oxide, barium oxide, etc, and particularly preferred ismagnesium oxide. The oxide is added preferably in an amount of 0.000001to 1.0 wt % of the entire oxide of the present invention, morepreferably 0.00001 to 0.1 wt %, and particularly preferably 0.0001 to0.01 wt %.

The sintering promoter includes, for example, alkaline earth metalcarbonates such as magnesium carbonate, as well as alumina, galliumoxide, titanium oxide, silicon oxide, germanium oxide, zirconium oxide,hafnium oxide, etc. In case where, for example, an alkaline earth metalcarbonate is used as the sintering promoter, the obtained oxide isoxidized by sintering and therefore contains an alkaline earth metaloxide derived from the promoter.

The content of the metal oxide other than the oxide of the alkalineearth metal is also preferably 0.00001 to 1.0 wt % relative to theentire oxide of the present invention, and more preferably 0.0001 to 0.1wt %.

In production of the oxide, the oxide single crystal and the ceramic ofthe present invention, some accessory constituents may mix therein; andfor example, constituent components of crucible may mix therein. Theoxide of the present invention does not exclude the contaminationthereof with such unexpected accessory constituents; however, its amountis no greater than 50 wt %, preferably no greater than 20 wt %, morepreferably no greater than 10 wt %, yet more preferably no greater than1 wt %, particularly preferably no greater than 0.1 wt %, mostpreferably no greater than 0.01 wt %, as a total with the othercomponents mentioned above.

The oxide of the present invention has a Verdet constant at a wavelengthof 1.06 mm of at least 0.18 min/(Oe·cm). Not specifically defined, theVerdet constant may be good to be at least 0.18 min/(Oe·cm); however,the oxide has a higher Verdet constant. When the Verdet constant is lessthan 0.18 min/(Oe·cm), then the Faraday rotator necessary to attain theFaraday rotation angle of 45° shall be long, and the optical isolatorshall be large-scaled.

Preferably, the Verdet constant is at least 0.20 min/(Oe·cm), morepreferably at least 0.21 min/(Oe·cm), yet more preferably at least 0.22min/(Oe·cm). From the viewpoint of the easiness in production, preferredis no greater than 0.36 min/(Oe·cm).

In the present invention, the Verdet constant may be determinedaccording to an ordinary method with no specific limitation thereon.

Concretely, the oxide having a given thickness is cut out, polished tohave a mirror face, and set with a permanent magnet having a knowndensity of magnetic flux, and its Verdet coefficient at a wavelength of1.06 mm is measured. The measurement is carried out at 25±10° C. in air.

The oxide of the present invention preferably has a transmittance (lighttransmittance) of at least 70% at a wavelength of 1.06 mm for an opticallength of 3 mm. When the transmittance is at least 70%, the transparencyis high and the oxide is favorable for use as a Faraday rotator.

The oxide of the present invention has a transmittance of at least 70%at a wavelength of 1.06 mm and for an optical length of 3 mm, preferablyat least 72%, more preferably at least 75%. The transmittance ispreferably higher, and not specifically limited, its uppermost limit maybe good to be at most 100%.

The transmittance is determined by the intensity of light having awavelength of 1.06 mm, as passed through the oxide having a thickness of3 mm. Concretely, the transmittance is represented by the followingformula:

Transmittance=I/I ₀×100,

(in the formula, I indicates the intensity of the transmitted light (theintensity of the light having passed through the sample having athickness of 3 mm); and I₀ indicates the intensity of the incidentlight.

In case where the transmittance of the obtained oxide is not uniform,and fluctuates in different sites analyzed, then the data of arbitrary10 points are averaged, and the resulting mean transmittance is thetransmittance of the oxide.

The oxide of the present invention preferably has a transmittance of atleast 70% at a wavelength of 1.06 mm and for an optical length of 3 mm,but more preferably has a high transmittance even for a long opticallength. Concretely, the transmittance for an optical length of 10 mm ispreferably at least 60%, preferably at least 70%, yet more preferably atleast 72%, particularly preferably at least 75%. The same shall apply tothe transmittance for an optical length of 15 mm, which is preferably atleast 60%, more preferably at least 70%, yet more preferably at least72%, particularly preferably at least 75%.

In case where the oxide of the present invention is used especially as aFaraday rotator, its transmittance for an optical length of 10 mm ispreferably at least 70%.

(Oxide Single Crystal, Ceramic)

The oxide of the present invention may be a single crystal or a ceramicand is not specifically limited, so far as it satisfies theabove-mentioned requirement. The case where the oxide of the presentinvention is an oxide single crystal, and the case where the oxide ofthe present invention is a ceramic are described in detail hereinunderalong with their production methods.

<Oxide Single Crystal>

The oxide of the present invention may be an oxide single crystal.Specifically, the oxide single crystal is an oxide single crystalcomprising the oxide of the present invention.

The method for forming an oxide crystal is not specifically limited, andincludes, for example, a floating zone melt method, a micro-pull downmethod, a pull up method, a skull melt method, and a Bridgman method.These methods are described in detail in “Newest Technology andApplication Development of Bulk Single Crystal” (edited by ShoseiFukuda, published by CMC, March 2006) and “Handbook of Crystal Growth”(edited by the Editorial Committee for “Handbook of Crystal Growth” ofthe Japanese Association for Crystal Growth, published by KyoritsuPublishing, September 1995).

In formation of the oxide single crystal, preferably, an alkaline earthmetal oxide (for example, magnesium, calcium, strontium, barium) isdoped for stable crystallization in an amount of 0.001 to 0.01 wt %, asdescribed above.

Typical production methods are described in detail hereinunder.

<Floating Zone Method>

One embodiment of forming an oxide single crystal according to afloating zone method is described.

For the floating zone method for producing a single crystal, forexample, referred to is JP-A-62-271385.

First, as starting materials, powdery materials (Tb₂O₃ and R₂O₃ andother components) having a high purity (preferably at least 99.9 wt %)are prepared and mixed to give a mixed powder. R contains at least oneelement selected from a group consisting of scandium, yttrium,lanthanum, europium, gadolinium, ytterbium, holmium and lutetium, and ispreferably selected from a group consisting of scandium, yttrium,lanthanum, europium, gadolinium, ytterbium, holmium and lutetium.

The mixed powder for use for production and the preparation of theshaped compact thereof are described below.

A xenon lamp floating zone method (xenon lamp FZ method), a type of anoptical floating zone method is described in detail with reference toFIG. 1.

Unless otherwise specifically indicated, the same reference numeralmeans the same object.

FIG. 1 is a conceptual cross-sectional view showing the constitution ofa xenon lamp FZ apparatus 100 for use in a xenon lamp FZ method. Thexenon lamp FZ apparatus 100 is so designed as to comprise a xenon lamp120 light source for melting, and an oval mirror 130, in which the ovalmirror 130 is formed by connecting two ovals to be endless, and thisacts to focus the light from the xenon lamp 120 toward the sample toheat and melt it. In FIG. 1, the xenon lamp FZ apparatus 100 is sodesigned that a hollow quartz tube 140 for putting a sample therein andtwo xenon lamps 120 are inside one oval mirror 130. Two ovals formingthe oval mirror 130 each have two focal points, and the oval mirror 130therefore has four focal points in total. Of the four focal points ofthe oval mirror 130, two focal points overlap with each other, and thequartz tube 140 is so arranged that it runs through the overlappingpoints. The axial cores of the two xenon lamps 120 are so arranged thatthey run through the remaining two focal points of the four focal pointsof the oval mirror 130.

The inner surface of the oval mirror 130 is mirror-finished. The xenonlight emitted by the xenon lamp 120 is reflected on the mirror-finishedoval mirror 130, and is led to come in the quartz tube 140 at the axialcore part nearly in every direction. As the light source, usable is ahalogen lamp in addition to the xenon lamp; however, the xenon lamp isadvantageous in that its ultimate temperature can be high and itslight-focusing degree can be sharp, and therefore the temperaturegradient can be steep.

The quartz tube 140 has a rotatable upper shaft 110 and a lower shaft112 as downwardly separated from the lower end of the upper shaft 110,inside the tube. The upper shaft 110 and the lower shaft 112 are movableup and down inside the quartz tube 140. Inside the quartz tube 140, theatmosphere for crystal growth is controllable. As a starting materialrod, a shaped compact of the starting material is fitted to the uppershaft 110. Preferably, a material of seed crystal is fitted to the lowershaft, but a shaped compact of the starting material or a sinteredcompact of the starting material may be fitted thereto. In this state, ashaped compact of the starting material fitted to the upper shaft isreferred to as a feed rod 114; and the shaped compact or the sinteredcompact of the starting material or the material as a seed crystalfitted to the lower shaft is referred to as a seed rod 116.

In FIG. 1, preferably, the quartz tube 140 is kept under positivepressure by introducing argon gas and a few % of hydrogen gas from oneend to the other end (not shown) thereinto. One reason for this is forprotecting the quartz tube 140 from being invaded by air from theoutside; and another reason is for protecting terbium oxide contained inthe starting material rod (feed rod 114) from being oxidized duringcrystal growth.

Subsequently, the feed rod 114 and the seed rod 116 are fitted to theupper and lower shafts 110 and 112, respectively, these are so arrangedthat their ends are kept adjacent to each other, and in that condition,the output of the xenon lamp 120 is elevated up to a temperature atwhich both the lower end of the feed rod 114 and the upper end of theseed rod 112 begin to melt. And then, the rods are moved closer to eachother while rotated reversely. These two rods need no rotation. In thiscondition, the two rods are kept in contact with each other to form amelt part. In this situation, while the output of the xenon lamp 120 isdelicately controlled, the seed rod 116 and the feed rod 114 aregradually let down so that the formed melt part could keep a suitablemelt form by the surface tension thereof. Accordingly, a crystal havinga predetermined composition is formed at the lower part of the meltpart, or that is, at the upper part of the seed rod 116. When thedescending speed of the seed rod 116 and that of the feed rod 114 aremade the same, then the crystal is grown. When the crystal is grown to apredetermined length or when the seed rod 116 is consumed, thedescending of the rod is stopped and the output of the xenon lamp 120 isgradually lowered to lower the temperature, whereby a transparentcrystal can be obtained.

In the floating zone method, the obtained crystal is grown under astrong temperature gradient condition, and therefore thermal strainduring the growth remains in the crystal; and during cutting, thecrystal may be cracked. Accordingly, after the crystal growth, it isdesirable that, using a carbon furnace, the crystal is put into a carboncontainer and annealed therein in an inert atmosphere or a reducingatmosphere at 1,200° C. or higher to remove the thermal strain. Theannealing temperature is not specifically limited, but is preferably1,200 to 2,200° C., more preferably 1,400 to 2,200° C., yet morepreferably 1,600 to 2,000° C. Also not specifically limited, theannealing time is preferably 1 to 100 hours, more preferably 5 to 50hours, yet more preferably 10 to 50 hours.

In case where the obtained single crystal is used as the Faraday rotatorof an isolator, preferably, after cutting, its surface ismirror-finished with an abrasive, etc. Not specifically limited, theabrasive may be, for example, colloidal silica.

<Micro-Pull Down Method>

As another method for forming an oxide single crystal, a micro-pull downmethod for forming a single crystal is described below. Regarding themicro-pull down method, referred to is JP-A-2001-226196.

First, the starting material powders are weighed in a desired ratio bymol. Before fed into the apparatus, the powdery starting materials arewell mixed, or may be good to be dried or sintered, for which any knownmethod is suitably employed. The method of preparing the mixed powder isdescribed below.

Using a micro-pull down apparatus, a single crystal is grown.

FIG. 2 is an explanatory view showing one example of the micro-pull downmethod favorably used as an embodiment.

The micro-pull down apparatus 200 for use in the micro-pull down methodis a single crystal growing apparatus that comprises a crucible 220, aseed holding tool 260 for holding the seed to be kept in contact withthe melt 210 flowing out from the pore formed through the bottom of thecrucible, a moving mechanism (not shown) for moving downward the seedholding tool 260, a moving speed controller (not shown) for the movingmechanism, and an induction heater 250 for heating the crucible 220. InFIG. 2, the lower part of the crucible 220 is supported by the cruciblesupporting tool 222, and an insulating jacket 230 and a quartz tube 240are provided outside the crucible 220, and the crucible 220 is thusheated by the induction heater 250 from the outside of the quartz tube240.

The crucible 220 is preferably formed of a rhenium metal sinteredcompact or a rhenium metal alloy sintered compact from the viewpoint ofthe heat resistance thereof, and preferably, an after heater (not shown)that is a heater formed of a rhenium metal sintered compact or a rheniummetal alloy sintered compact is arranged around the outer periphery ofthe bottom of the crucible. The heat value of the crucible 220 and theafter heater can be controlled by controlling the output of theinduction heater 250, whereby the heating temperature and thetemperature profile of the solid-liquid interface of the melt 210 to bedrawn out through the pore formed through the bottom of the crucible canbe controlled.

In this apparatus, preferably, multiple fine pores are provided eachhaving a size through which the melt does not drop down (preferablyhaving a diameter of 200 to 300 μm), and the falling melts through thefine pores could join together before they are brought into contact withthe seed crystal or the sintered compact formed by shaping a sinteredstarting material having the same composition.

Using this apparatus, the sintering material prepared according to theabove-mentioned method is set in the crucible 220. Before heating,preferably, the furnace is made to have an inert gas atmosphere insideit, and by gradually applying a high-frequency power to thehigh-frequency induction heating coil (induction heater 250), thecrucible 220 is thereby heated and the material inside the crucible 220is completely melted. If possible, this state is preferably maintainedfor a few hours in order that the melt 210 could have a uniformcomposition.

The seed crystal or the sintered shaped rod is gradually elevated at apredetermined speed, and its top is kept in contact with the fine poreof the bottom of the crucible and is thereby well wetted with the melt.Subsequently, with the temperature of the melt kept controlled, the pulldown axis is let down to thereby make the crystal grow. At the pointwhen the prepared materials have been all crystallized and the melt hasdisappeared, the crystal growth is finished. The grown crystal is, whilekept on an after heater, preferably gradually cooled down to roomtemperature.

(Ceramic (Transparent Ceramic))

The solid solution does not have to be a single crystal so far as it ishighly transparent at a wavelength of 1.06 mm and is free fromanisotropy such as thermal strain, etc., and may be a polycrystallineceramic (in the present invention, this may be referred to as atransparent ceramic). In the present invention, the transparent ceramicmeans a ceramic having a transmittance of at least 70% at a wavelengthof 1.06 mm and for an optical length of 3 mm.

In case where a single crystal is produced, the system must be heated upto a high temperature so as to form a melt state. Terbium oxide has amelting point of about 2,600° C., yttrium oxide has a melting point ofabout 2,300° C.; and when the two oxides form a solid solution, theymust be heated up to the intermediate temperature of the two meltingpoints, or that is, they must be heated up to an extremely hightemperature. Accordingly, in case where a single crystal is formed bymelting in a crucible, the material of the crucible to be selected isextremely limited to rhenium, tungsten or their alloy, etc.

On the other hand, a transparent ceramic does not need heating up to itsmelting point, but can be made transparent at a temperature not higherthan the melting point thereof so far as it is sintered under pressure.During sintering, a sintering promoter may be added to increase thesintering density to thereby make the sintered ceramic densified.

The method for forming the transparent ceramic is not specificallylimited, and any conventionally known method may be suitably selectedand employed. The production method for transparent ceramics includes ahot isotactic pressing method, a combination of a solid phase method anda press forming method, a method of vacuum sintering by die casting,etc., which are described in Akio Ikesue, “From Optical Single Crystalto Optical Polycrystal”, Applied Physics, Vol. 75, No. 5, pp. 579-583(2006); Takahiro Yanagiya & Hideki Yagi, “Current State and FutureProspects of Ceramic Laser Materials” Laser Studies, Vol. 36, No. 9, pp.544-548 (2008), etc.

As a production method for a transparent ceramic, hereinunder describedis one example of a hot isostatic pressing (HIP) method for producing atransparent ceramic.

First, a mixed powder of starting material powders (Tb₂O₃ and R₂O₃ andother components) are prepared and mixed to give a mixed powder. Themethod for preparing the mixed powder is described below. A solvent, abinder, a plasticizer, a lubricant and others are added to the obtainedmixed powder, and wet-mixed to be slurry. In this state, theabove-mentioned sintering promoter may be added in a predeterminedamount, preferably in an amount of 0.00001 to 1.0 wt % of the totalamount of all the starting materials, more preferably 0.0001 to 0.1 wt%, yet more preferably 0.001 to 0.01 wt %. The obtained slurry isprocessed with a spray drier and dried, and thereafter this is shaped.The shaping may be attained in one stage or in multiple stages. Aftershaped, preferably this may be degreased by heating (preferably at 400to 600° C.).

Subsequently, this is preferably sintered in a vacuum furnace. Regardingthe sintering condition, the temperature is preferably 1,600 to 2,000°C., more preferably 1,700 to 1,900° C., yet more preferably 1,750 to1,850° C. The sintering time is preferably 1 to 50 hours, morepreferably 2 to 25 hours, yet more preferably 5 to 20 hours. In thisstage, the heating speed is preferably 100 to 500° C./hr up to around1,200° C. or so, more preferably 200 to 400° C./hr, yet more preferably250 to 350° C./hr; and at a temperature higher than it, the heatingspeed is preferably lowered to be 25 to 75° C./hr. The vacuum degree insintering is preferably at most 1 Pa, more preferably at most 1×10⁻¹ Pa.

After thus sintered, this is processed according to a hot isotropicpressing (HIP) method for further increasing the transparency thereof.The processing temperature is preferably higher than the sinteringtemperature, and is preferably 1,600 to 2,000° C., more preferably yet1,700 to 1,900° C., yet more preferably 1,750 to 1,850° C. Theprocessing pressure is preferably 10 to 1,000 MPa, more preferably 20 to500 MPa, yet more preferably 40 to 200 MPa. The processing time is notspecifically limited, but is preferably no greater than 50 hours, morepreferably no greater than 25 hours, yet more preferably no greater than10 hours. Also preferably, the time is at least 15 minutes, morepreferably at least 30 minutes, yet more preferably at least 1 hour.

<Preparation of Mixed Powder and Shaped Compact>

In the present invention, the starting materials for the mixed powderand its shaped compact to be used in production of the oxide singlecrystal and the transparent ceramic are weighed in a desired molarratio.

The powdery materials (Tb₂O₃, R₂O₃, and other components) for use hereinare preferably of high-purity, having a purity of at least 99.9 wt %,more preferably at least 99.99 wt %, yet more preferably at least 99.999wt %. R in R₂O₃ has the same meaning as that of R in Formula (I), andits preferred range is also the same.

Terbium oxide is not limited to Tb₂O₃, and Tb₄O₇ may also be used.However, use of Tb₂O₃ is preferred since the crystallinity of theobtained oxide is excellent.

The powdery materials are weighed in a desired molar ratio, thendry-mixed or wet-mixed with no specific limitation thereon. After thuswet or dry-mixed, the mixture may be sintered; or the mixture may besintered and further ground.

Concretely, after the materials are dry-mixed with a ball mill, etc.,the mixed powder is sintered in an inert gas atmosphere. This method maybe referred to herein as one example. The sintering temperature and thesintering time are not specifically limited. The sintering temperatureis preferably 600 to 2,000° C., more preferably 800 to 1,800° C., yetmore preferably 1,000 to 1,800° C. The inert gas atmosphere includes arare gas atmosphere, a nitrogen gas atmosphere, etc.; preferably,however, the mixed powder is sintered in an argon atmosphere. Thesintering time is not specifically limited, but may be suitably selecteddepending on the water content of the mixed powder and the sinteringtemperature. The sintering time is preferably 1 to 100 hours, morepreferably 5 to 50 hours, yet more preferably 10 to 30 hours. Aftersintered, the material is preferably ground and mixed in a ball mill,etc.

For the purpose of sharpening the mean particle size distribution of themixed powder and for the purpose of making the mixed powder have a highpurity, the powdery materials may be melted, recrystallized and ground,and then used as starting material powders.

Concretely, starting material powders having a high purity (for example,at least 99.9%) are prepared, and are so weighed that Tb₂O₃/R₂O₃ thereincould be a desired ratio by mol. These starting material powders aredissolved to prepare an aqueous nitric acid solution having aconcentration of 1 mol/l, and an aqueous ammonium sulfate solutionhaving a concentration of 1 mol/l is mixed therein, and furtherultrapure water was added, the concentration was controlled, and withthe resulting aqueous solution kept stirred, an aqueous ammoniumhydrogencarbonate solution having a concentration of 0.5 mol/l wasdropwise added at a constant addition rate until the system could have apH of 8, and with stirring, this was left at room temperature for a fewdays, and thereafter filtered and washed with ultrapure water, and driedat 150° C. for a few days. This method is employable here as oneexample. The obtained mixed powder is put into an alumina crucible, andcalcined in an inert atmosphere such as a nitrogen atmosphere, an argonatmosphere, etc., preferably at 800 to 1,500° C., more preferably at1,000 to 1,400° C., yet more preferably at 1,100 to 1,200° C., andpreferably for 0.5 to 10 hours, more preferably for 1 to 7 hours, yetmore preferably for 2 to 4 hours. In this state, the inert atmosphere isemployed for preventing the valence of terbium oxide from changing.

After the powdery materials are well mixed, the mixture may be shaped tohave a desired shape and size, using a shaping machine. The shape to beformed is not specifically limited, and may be suitably selecteddepending on the apparatus to be used. For example, the mixture may beshaped to be columnar.

One example of the shaping method for the powdery materials comprises,for example, well dry-mixing the starting material powders and shapingthe resulting mixture under pressure using a shaping machine.

An organic binder may be added to make the powdery material into aslurry state; or after this state is shaped and sintered to give asintered compact, and this may be used as a shaped compact of thestarting material. The sintering temperature is preferably 600 to 2,000°C., more preferably 800 to 1,800° C., yet more preferably 1,000 to1,800° C. The sintering atmosphere is preferably a rare gas or inert gasatmosphere, more preferably an argon atmosphere. The sintering time isnot specifically limited, but is preferably for 1 to 100 hours, morepreferably for 5 to 50 hours, yet more preferably for 10 to 30 hours.

In case where a transparent ceramic is produced according to a HIPmethod, a shaped compact is first produced and this is processedaccording to a HIP method.

A concrete production method for a shaped compact comprises adding asolvent, a binder, a plasticizer, a lubricant and others to a startingmaterial powder, and wet-mixing them to be slurry. In this state, apredetermined amount of a sintering promoter may be added. Theproduction method for the shaped compact is not specifically limited.For example, the obtained slurry may be processed with a spray drier togive dry spheres.

The solvent to be used for the slurry is not specifically limited. Fromthe viewpoint of the easiness in handling, preferred is water or a loweralcohol; more preferred is water, methanol or ethanol; and yet morepreferred is methanol. Not specifically limited, the binder may be anyone suitably selected from known binders, and its one example ispolyvinyl alcohol.

The plasticizer and the lubricant are not also specifically limited, andmay be suitably selected from known plasticizers and lubricants. Oneexample of the plasticizer is polyethylene glycol; and one example ofthe lubricant is stearic acid.

The dried spheres are, after shaped, preferably degreased. The shapingmethod is not specifically limited, and may be suitably selected fromany known shaping methods. The shaping may be attained in one stage orin multiple stages.

The degreasing is preferably carried out by heating. The heatingtemperature is preferably from 400 to 600° C. In degreasing, the heatingup to 400° C. may be attained in air, but at a temperature higher thanthis temperature the heating is carried out preferably in an inertatmosphere.

(Magneto-Optical Material)

The oxide, the oxide single crystal and the ceramic of the presentinvention are suitable for use in magneto-optical materials. Inparticular, the oxide, the oxide single crystal and the ceramic of thepresent invention are suitably used as a Faraday rotator for an opticalisolator at a wavelength range of 0.9 to 1.1 μm.

FIG. 3 is a schematic cross-sectional view showing one example of anoptical isolator that is an optical device having a Faraday rotator asan optical element.

In FIG. 3, an optical isolator 300 includes a Faraday rotator 310, and apolarizer 320 and an analyzer 330, which are polarizing materials, areprovided in front of and to the rear of the Faraday rotator 310.Furthermore, with regard to the optical isolator 300, the polarizer320-Faraday rotator 310-analyzer 330 are disposed in this order on anoptical axis 312, a magnet 340 is placed on at least one side of thesides thereof, and the magnet 340 is preferably housed in the interiorof an enclosure 350.

The isolator is preferably used for a fiber laser for a processingmachine. Specifically, it is suitably used to prevent the oscillationfrom being unstable by returning the laser light emitted by the laserelement to the element.

EXAMPLES

The present invention is further described with reference to Examplesand Comparative Examples; however, the present invention should not belimited to the following Examples.

Examples 1 to 6, Comparative Examples 1 to 3

Powdery starting materials of Tb₂O₃ having a high purity of at least99.9 wt % and Y₂O₃ having a purity of at least 99.9 wt % were prepared,and these were weighed in a desired molar ratio of Tb₂O₃/Y₂O₃.Subsequently, the starting material composition was well mixed, andusing a shaping machine, the mixture was shaped into a columnar compacthaving a diameter of 3 mm and a length of 50 mm.

Subsequently, using a xenon lamp FZ apparatus as shown in FIG. 1, asingle crystal was grown.

The quartz tube 140 was once processed for drying and deoxygenation, andthen, made to have a positive pressure by introducing argon gas and 8%of hydrogen gas from one end to the other end (not shown) thereinto. Onereason for this is for protecting the quartz tube from being invaded byair from the outside; and another reason is for protecting terbium oxidecontained in the starting material rod from being oxidized duringcrystal growth.

The above-mentioned, shaped compacts of the starting material bothhaving the same composition and having a size of 3 mm diameter

mm length were fitted to the upper and lower shafts, these were soarranged that their ends could be kept adjacent to each other, and inthat condition, the output of the xenon lamp was elevated up to atemperature at which both the lower end of the feed rod and the upperend of the seed rod began to melt. With that, the rods were moved closerto each other while rotated reversely. In this condition, the two rodswere kept in contact with each other to form a melt part. In thismoment, while the output of the xenon lamp was delicately controlled,the seed rod and the feed rod were gradually let down at a speed of 8mm/hr so that the formed melt part could keep a suitable melt form bythe surface tension thereof. Accordingly, a crystal having apredetermined composition was formed at the lower part of the melt part,that is, at the upper part of the seed rod. The descending speed of theseed rod and that of the feed rod were made to be the same, and thecrystal having a diameter of 3 mm was thus grown. When the crystal wasgrown to a length of 30 mm, the descending rods were stopped and theoutput of the xenon lamp was gradually lowered, (taking about 1 hour orso), to thereby lower the temperature to give a transparent crystal.

After thus grown, the crystal was put into a vacuum heat treatmentfurnace, and annealed in an argon atmosphere at 1,600° C. therein for 15hours to remove the thermal strain.

The annealed solid solution single crystal having a size of 3 mmdiameter×30 mm length was trimmed at their both edges using an internalperiphery blade cutter, and both edges were polished with an abrasivesuch as colloidal silica, etc. thereby mirror-finish them. The Verdetconstant of the cylindrical crystal thus obtained having a size of 3 mmdiameter×25 mm length was determined. The results of Examples andComparative Examples are shown in Table 1.

In Formula (I), when 0.4≦x≦1.0, the Verdet constant at a wavelength of1.06 μm was at least 0.18 min/(Oe·cm). This is substantially twice ormore the size of the Verdet constant of a TGG crystal, which is 0.13min/(Oe·cm).

Measurements for the transmittance (%) and the extinction ratio (dB) foran optical path length of 3 mm are shown. Transmittance and extinctionratio were measured in a state in which there was no nonreflectivecoating.

In Example 6, an alkaline earth metal oxide, MgO was added for thepurpose of more stabilizing the crystallization. MgO was added asfollows: Tb₂O₃ and Y₂O₃ were weighed in a desired molar ratio ofTb₂O₃/Y₂O₃, then a predetermined amount of MgO was added thereto, andthe starting material mixture was well mixed. Using a shaping machine,the mixture was shaped into a columnar compact having a size of 3 mmdiameter×50 mm length.

TABLE 1 MgO Extinction Verdet Tb₂O₃ Y₂O₃ Parts by Transmittance ratioconstant mol % mol % weight % dB min/Oe · cm Example 1 0.4 0.6 0 79 290.18 Example 2 0.5 0.5 0 77 30 0.23 Example 3 0.6 0.4 0 78 30 0.27Example 4 0.7 0.3 0 71 27 0.31 Example 5 0.8 0.2 0 78 26 0.33 Example 60.8 0.2 0.001 75 26 0.34 Comparative 0.1 0.9 0 80 30 0.05 Example 1Comparative 0.2 0.8 0 80 29 0.09 Example 2 Comparative 0.3 0.7 0 79 290.13 Example 3 (Note) The Verdet constant is the value at a wavelengthof 1.06 μm; the transmittance and extinction ratio are values without anonreflective coating.

Examples 7 to 12, Comparative Examples 4 to 9

Subsequently, of rare earth oxides such as scandium, lanthanum,europium, gadolinium, ytterbium, holmium, lutetium, etc., the results ofthe solid solution single crystals of gadolinium oxide or lutetium oxideand terbium oxide are shown. The production method for the oxide singlecrystals was the same as in Example 1, except that Gd₂O₃ or Lu₂O₃ wasused in place of Y₂O₃.

TABLE 2 Verdet Extinction constant Tb₂O₃ Gd₂O₃ Transmittance ratio min/mol % mol % % dB Oe · cm Example 7 0.4 0.6 79 29 0.19 Example 8 0.5 0.579 30 0.24 Example 9 0.6 0.4 76 30 0.26 Comparative 0.1 0.9 80 29 0.04Example 4 Comparative 0.2 0.8 79 29 0.08 Example 5 Comparative 0.3 0.780 28 0.13 Example 6 (Note) The Verdet constant is the value at awavelength of 1.06 μm; the transmittance and extinction ratio are valueswithout a nonreflective coating.

TABLE 3 Verdet Extinction constant Tb₂O₃ Lu₂O₃ Transmittance ratio min/mol % mol % % dB Oe · cm Example 10 0.4 0.6 79 30 0.19 Example 11 0.50.5 79 31 0.24 Example 12 0.6 0.4 75 31 0.26 Comparative 0.1 0.9 80 300.04 Example 7 Comparative 0.2 0.8 80 29 0.08 Example 8 Comparative 0.30.7 79 29 0.11 Example 9 (Note) The Verdet constant is the value at awavelength of 1.06 μm; the transmittance and extinction ratio are valueswithout a nonreflective coating.

Examples 13 to 19 and Comparative Examples 10 to 12

A single crystal was grown, using a micro-pull down apparatus as in FIG.2. Herein used was a single crystal growing apparatus comprising arhenium crucible having a diameter of 20 mm, a seed holding tool forholding the seed to be kept in contact with the melt flowing out fromthe fine pore formed through the bottom of the rhenium crucible, amoving mechanism for moving downward the seed holding tool, a movingspeed controller for the moving mechanism, and an induction heater forheating the crucible. In addition, an after heater formed of rhenium wasarranged. Two or three fine pores each having a diameter of 200 mm wereformed through the bottom of the crucible.

Powdery starting materials of Tb₂O₃ having a purity of at least 99.9 wt% and Y₂O₃ having a purity of at least 99.9 wt % were prepared, andthese were weighed in a predetermined molar ratio of Tb₂O₃/Y₂O₃.Subsequently, pure water was added to the starting material composition,wet-mixed for 3 hours, and the mixed powder was dewatered andvacuum-dried. Subsequently, the powder was ground, then ethanol andethylene glycol were added thereto and wet-mixed to be slurry. Theslurry mixture was shaped into a columnar compact having a size of 3 mmdiameter×50 mm length, using a shaping machine. The shaped compact wassintered in an argon atmosphere at 1,600° C. for 2 hours to give aceramic sintered compact having a size of 3 mm diameter×50 mm length.

Using a micro-pull down apparatus, the sintered material, as driedaccording to the above-mentioned method, was set in a crucible. Beforeheating, the furnace was degassed in vacuum, then argon having a purityof 99.99% was introduced thereinto, whereby the furnace was made to havean inert gas atmosphere. A high-frequency electric power was graduallygiven to the high-frequency induction heating coil to thereby heat thecrucible so that the material in the crucible was completely melted.This state was kept as such for 8 hours so that the melt compositioncould be uniform.

The ceramic sintered compact having a size of 3 mm diameter

mm length was gradually elevated at a predetermined speed, and its topwas kept in contact with the fine pore of the bottom of the crucible andwas thereby well wetted with the melt. Subsequently, with thetemperature of the melt kept controlled, the pull down axis was let downto thereby make the crystal grow. At the point when the preparedmaterials were all crystallized and the melt disappeared, the crystalgrowth was finished. The grown crystal was, while kept on an afterheater, gradually cooled to room temperature.

The obtained crystal was grown under a strong temperature gradientcondition, and therefore thermal strain during the growth remained inthe crystal; and when cut, the crystal would be cracked. Accordingly,after the crystal growth, the crystal was put in a vacuum heat treatmentfurnace, and annealed in an argon atmosphere at 1,800° C. for 12 hoursto remove the thermal strain.

Thus annealed, the oxide single crystal having a size of 3 mmdiameter×30 mm length was trimmed at its both edges using an internalperiphery blade cutter, and both edges were polished with an abrasivesuch as colloidal silica, etc. to thereby mirror-finish them. Thusobtained, the Verdet constant of the cylindrical crystal having a sizeof 3 mm diameter×25 mm length was determined. The results of Examplesand Comparative Examples are shown in Table 4. When the molar ratio ofTb₂O₃/Y₂O₃ is at least 0.4/0.6, the Verdet constant was at least 0.18min/(Oe·cm). This is nearly at least two times the Verdet constant, 0.13min/(Oe·cm) of a TGG crystal.

TABLE 4 Verdet Extinction constant Tb₂O₃ Y₂O₃ Transmittance ratio min/mol % mol % % dB Oe · cm Example 13 0.4 0.6 79 29 0.20 Example 14 0.50.5 79 29 0.22 Example 15 0.6 0.4 78 31 0.24 Example 16 0.7 0.3 77 270.28 Example 17 0.8 0.2 72 29 0.32 Example 18 0.9 0.1 75 28 0.36 Example19 1.0 0.0 70 26 0.40 Comparative 0.1 0.9 80 29 0.06 Example 10Comparative 0.2 0.8 79 28 0.09 Example 11 Comparative 0.3 0.7 80 29 0.13Example 12 (Note) The Verdet constant is the value at a wavelength of1.06 μm; the transmittance and extinction ratio are values without anonreflective coating.

Examples 20 to 25 and Comparative Examples 13 to 15

These Examples and Comparative Examples are to demonstrate theproduction of ceramics (transparent ceramics) according to a hotisotactic pressing method for producing transparent ceramics.

First, powdery starting materials of Tb₂O₃ having a high purity of 99.9and Y₂O₃ having a purity of 99.999% were prepared, and these wereweighed in a predetermined molar ratio of Tb₂O₃/Y₂O₃. The Tb₂O₃ powderand the Y₂O₃ powder were mixed in a predetermined molar ratio, and themixed powder was dissolved in an aqueous nitric acid solution having aconcentration of 1 mol/l. Aqueous ammonium sulfate solution having aconcentration of 1 mol/l was mixed therein, then ultrapure water wasadded, and the concentration of the solution was controlled. With theresulting aqueous solution kept stirred, an aqueous ammoniumhydrogencarbonate solution having a concentration of 0.5 mol/l wasdropwise added at a constant addition rate until the system could have apH of 8, and with stirring, this was left at room temperature for 2days, and thereafter filtered and washed with ultrapure water, and driedat 150° C. for 2 days. The obtained mixed powder was put into an aluminacrucible, and calcined in an inert atmosphere such as a nitrogenatmosphere, an argon atmosphere, etc. in an electric furnace at 1,200°C. for 3 hours. The inert atmosphere was employed for preventing thevalence of terbium oxide from changing.

100 g of the starting material powder prepared in the above, 50 g ofmethanol as a solvent, 1 g of polyvinyl alcohol as a binder, 1 g ofpolyethylene glycol as a plasticizer, and 0.5 g of stearic acid as alubricant were wet-mixed in a nylon ball mill to be slurry. In this, apredetermined amount, for example, 0.001 to 0.01 parts by weight of asintering promoter was added to the mixture.

The obtained slurry was processed with a spray drier to give dryspheres. The dry spheres were put into a 5 mmφ mold, primary-shapedtherein, and then further shaped according to a cold isotactic press(CIP) method under a pressure of 200 MPa. The shaped compact isdegreased at an elevated temperature of 400 to 600° C. Up to 400° C.,the compact was heated in air, and at a higher temperature, the compactwas heated in an inert atmosphere.

Subsequently, this was sintered in a vacuum furnace at 1,700° C. for 8to 10 hours. The sintering condition was as follows. Up to 1,200° C.,the heating speed was 300° C./hr, and at a higher temperature, theheating speed was 50° C./hr. The vacuum degree was 0.5×10⁻¹ Pa.

For further increasing the transparency thereof, the treatment wasprocessed according to a hot isotactic press (HIP) method at 1,800° C.and under a pressure of 100 MPa for 10 hours.

The annealed ceramics having a size of 3 mm diameter×30 mm length wastrimmed at their both edges using an internal periphery blade cutter,and both edges were polished with an abrasive such as colloidal silica,etc. to thereby mirror-finish them. Thus obtained, the Verdet constantof the cylindrical ceramics having a size of 3 mmφ×25 mm was determined.The results of Examples and Comparative Examples are shown in Table 4.When the molar ratio of Tb₂O₃/Y₂O₃ is at least 0.4/0.6, the Verdetconstant was at least 0.18 min/(Oe·cm). This is nearly at least twotimes the Verdet constant, 0.13 min/(Oe·cm) of a TGG crystal.

TABLE 5 MgO Extinction Verdet Tb₂O₃ Y₂O₃ Parts by Transmittance ratioconstant mol % mol % weight % dB min/Oe · cm Example 20 0.4 0.6 0 80 300.18 Example 21 0.5 0.5 0 79 30 0.23 Example 22 0.6 0.4 0 80 30 0.24Example 23 0.7 0.3 0 75 28 0.27 Example 24 0.7 0.3 0.001 79 27 0.29Example 25 0.7 0.3 0.0005 77 26 0.28 Comparative 0.1 0.9 0 80 28 0.05Example 13 Comparative 0.2 0.8 0 80 29 0.09 Example 14 Comparative 0.30.7 0 80 29 0.13 Example 15 (Note) The Verdet constant is the value at awavelength of 1.06 μm; the transmittance and extinction ratio are valueswithout a nonreflective coating.

Examples 26 to 49 and Comparative Examples 16 to 26

In the same manner as in Example 19 except that the sintering promoterwas changed while Tb₂O₃/Y₂O₃=0.6/0.4 was kept constant as such, thesamples were evaluated for the transmittance, the extinction ratio andthe Verdet constant thereof. The results are shown in the followingTables 6 to 9.

TABLE 6 Al₂O₃ GeO₃ TiO₂ Extinction Verdet Tb₂O₃ Y₂O₃ Parts by Parts byParts by Transmittance ratio constant mol % mol % weight weight weight %dB min/Oe · cm Example 26 0.6 0.4 0.001 0 0 79 29 0.24 Example 27 0.60.4 0 0.001 0 80 28 0.25 Example 28 0.6 0.4 0 0 0.001 80 27 0.22 Example29 0.6 0.4 0.0005 0.0005 0.0005 80 28 0.22 Example 30 0.6 0.4 0.009 0 080 27 0.21 Example 31 0.6 0.4 0 0.009 0 80 26 0.21 Example 32 0.6 0.4 00 0.009 79 28 0.20 Example 33 0.6 0.4 0.009 0.009 0.009 74 26 0.20Comparative 0.6 0.4 1.2 0 0 68 24 0.22 Example 16 Comparative 0.6 0.4 01.2 0 67 22 0.21 Example 17 Comparative 0.6 0.4 0 0 1.2 65 21 0.22Example 18 (Note) The Verdet constant is the value at a wavelength of1.06 μm; the transmittance and extinction ratio are values without anonreflective coating.

TABLE 7 Nb₂O₃ Mo₂O₃ YF₃ Extinction Verdet Tb₂O₃ Y₂O₃ Parts by Parts byParts by Transmittance ratio constant mol % mol % weight weight weight %dB min/Oe · cm Example 34 0.6 0.4 0.001 0 0 76 28 0.23 Example 35 0.60.4 0 0.001 0 78 27 0.25 Example 36 0.6 0.4 0 0 0.001 79 28 0.23 Example37 0.6 0.4 0.5 0 0 78 28 0.22 Example 38 0.6 0.4 0 0.3 0 77 27 0.20Example 39 0.6 0.4 0 0 0.8 76 26 0.21 Comparative 0.6 0.4 1.1 0 0 66 220.22 Example 19 Comparative 0.6 0.4 0 1.1 0 62 21 0.21 Example 20Comparative 0.6 0.4 0 0 1.1 60 20 0.21 Example 21 (Note) The Verdetconstant is the value at a wavelength of 1.06 μm; the transmittance andextinction ratio are values without a nonreflective coating.

TABLE 8 Cu₂O₃ ZnO Bi₂O₃ Extinction Verdet Tb₂O₃ Y₂O₃ Parts by Parts byParts by Transmittance ratio constant mol % mol % weight weight weight %dB min/Oe · cm Example 40 0.6 0.4 0.0001 0 0 75 28 0.23 Example 41 0.60.4 0 0.0001 0 77 27 0.24 Example 42 0.6 0.4 0 0 0.00002 79 28 0.22Example 43 0.6 0.4 0.9 0 0 77 28 0.21 Example 44 0.6 0.4 0 0.9 0 75 270.20 Example 45 0.6 0.4 0 0 0.9 74 26 0.21 Comparative 0.6 0.4 1.2 0 065 23 0.20 Example 22 Comparative 0.6 0.4 0 1.1 0 61 22 0.21 Example 23Comparative 0.6 0.4 0 0 1.1 61 21 0.20 Example 24 (Note) The Verdetconstant is the value at a wavelength of 1.06 μm; the transmittance andextinction ratio are values without a nonreflective coating.

TABLE 9 SeO₃ TeO₂ Extinction Verdet Tb₂O₃ Y₂O₃ Parts by Parts byTransmittance ratio constant mol % mol % weight weight % dB min/Oe · cmExample 46 0.6 0.4 0.0001 0 74 28 0.24 Example 47 0.6 0.4 0 0.0001 76 270.21 Example 48 0.6 0.4 0.9 0 74 28 0.22 Example 49 0.6 0.4 0 0.9 75 270.20 Comparative 0.6 0.4 1.2 0 68 23 0.21 Example 25 Comparative 0.6 0.40 1.1 65 22 0.20 Example 26 (Note) The Verdet constant is the value at awavelength of 1.06 μm; the transmittance and extinction ratio are valueswithout a nonreflective coating.

Example 50

The produced (Tb_(0.6)Y_(0.4))₂O₃ crystal having 5 mmφ was finished tohave an outer diameter of 4.5 mmφ, and then sliced with an innerperiphery blade slicer. Its both edges were lapped with SiC abrasivegrains and polished with colloidal silica, thereby having a final lengthof 12 mm to give a Faraday rotator. Its length was enough to obtain arotational angle of 45° at a wavelength of 1.06 μm. The transmittance ata wavelength of 1.06 mm and an optical length of 12 mm was 70%.

Both faces of the Faraday rotator were coated with a non-reflective coatfor air.

On the other hand, two polarization beam splitter having a size of 10mm×10 mm square were prepared to be a polarizer and an analyzer for anoptical isolator. Both surfaces of these polarizer and analyzer werecoated with a non-reflective coat for air.

The Faraday rotator, the polarizer and the analyzer were built in ametal housing as combined therein. With a laser beam made to passthrough the center, the polarizer (or the analyzer) was rotated andregulated so that the reversed direction insertion loss could be themaximum, and thereafter the members were bonded and fixed. In thismoment, a permanent magnet was arranged around the outer periphery ofthe Faraday rotator. The optical device was set in a saturated magneticfield and its optical properties were measured. The reversed directioninsertion loss was 43 dB, and the regular direction insertion loss was0.20 dB. The isolator had a smaller insertion loss as compared withconventional devices, and exhibited high performance as an opticalisolator. In addition, as compared with that in conventional devices,the length of the Faraday rotator is short, that is, the isolator is adownsized optical isolator.

Description of the Reference Numerals in Figures is made as follows:

-   100 Xenon Lamp FZ Apparatus-   110 Upper Shaft-   112 Lower Shaft-   114 Feed Rod-   116 Seed Rod-   120 Xenon Lamp-   130 Oval Mirror-   140 Quartz Tube-   200 Micro-Pull Down Apparatus-   210 Melt-   220 Crucible-   222 Crucible Supporting Tool-   230 Insulating Jacket-   240 Quartz Tube-   250 Induction Heater-   260 Seed Holder-   300 Optical Isolator-   310 Faraday Rotator-   320 Polarizer-   330 Analyzer-   340 Magnet-   350 Enclosure

1.-10. (canceled)
 11. A magneto-optical material comprising an oxiderepresented by Formula (I) below at a content of at least 99 wt %,(Tb_(x)R_(1-x))₂O₃  (I) wherein in Formula (I), x satisfies 0.4≦x≦1.0and R comprises at least one element selected from the group consistingof scandium, yttrium, and lanthanoid elements other than terbium. 12.The magneto-optical material according to claim 11, wherein it has aVerdet constant at a wavelength of 1.06 μm of at least 0.18 min/(Oe·cm),a transmittance for an optical path length of 3 mm at a wavelength of1.06 μm of at least 70%, and an extinction ratio at an optical pathlength of 3 mm of at least 25 dB.
 13. The magneto-optical materialaccording to claim 11, wherein in Formula (I), R is selected from thegroup consisting of scandium, yttrium, lanthanum, europium, gadolinium,ytterbium, holmium, and lutetium.
 14. The magneto-optical materialaccording to claim 11, wherein it comprises an oxide of an alkalineearth metal, Group 11 element, Group 12 element, Group 13 element, Group14 element, Group 15 element, Group 16 element, Group 4 element, Group 5element, or Group 6 element or a compound of a Group 17 element at acontent of at least 0.00001 wt % but no greater than 1.0 wt %.
 15. Themagneto-optical material according to claim 11, wherein it comprises anoxide of an alkaline earth metal at a content of at least 0.00001 wt %but no greater than 1.0 wt %.
 16. The magneto-optical material accordingto claim 11, wherein it is a single crystal.
 17. The magneto-opticalmaterial according to claim 16, wherein the single crystal is producedby a production method selected from the group consisting of a floatingzone melting method, a micro-pulling-down method, a pulling up method, askull melting method, the Bridgman method, the Bernoulli method, and theEFG method.
 18. The magneto-optical material according to claim 11,wherein it is a ceramic.
 19. A Faraday rotator for a wavelength of atleast 0.40 μm but no greater than 1.2 μm employing the magneto-opticalmaterial according to claim
 11. 20. An optical isolator comprising theFaraday rotator according to claim 19 and a polarizing material placedin front of and to the rear of the Faraday rotator.